1. The wandering
Entry 858 concluded that a credible minimoon tug is a hybrid: a large solar sail for the bulk heliocentric energy change and a solar-electric propulsion (SEP) system for the directions and eclipses the sail cannot handle. This entry asks the next obvious question: how big and heavy would such a thing be for a 100 t rock?
This is a sketch, not a design. The goal is an order-of-magnitude budget that tells us whether the tug is a small accessory or a second payload.
2. Assumptions
- Target rock mass: 100 t.
- Required heliocentric Δv to move from a plausible minimoon orbit to an Earth-Moon intercept: somewhere between 2 and 4 km/s.
- SEP: Hall or ion thrusters with Isp ≈ 3,000 s and thruster-plus-PPU efficiency ≈ 65%.
- SEP dry specific mass: 30 kg/kW — optimistic but in the family of high-power systems like NASA’s Power and Propulsion Element.
- Sail: 1 km² of membrane, with an areal density of about 15 g/m². DLR’s GOSSAMER scaling suggests membrane around 10 g/m² and boom/deployment overhead around a few grams per square metre more.
- Bus, power, communications, attitude control, capture tether, and contingency: a round 15 t, treated as a placeholder.
3. The pieces
Sail
At 1 AU a perfect reflector sees about 9 µN/m² of radiation pressure. A 1 km² sail therefore delivers roughly 9 N of thrust when facing the Sun. At 15 g/m², that sail masses:
$$10^6 , \text{m}^2 \times 0.015 , \text{kg/m}^2 = 15, \text{t}$$
That is already heavier than the entire NEA Scout spacecraft and comparable to a small GEO satellite.
Solar-electric propulsion
A 250 kW SEP system at 30 kg/kW has a dry mass of about 7.5 t. With Isp = 3,000 s and 65% efficiency it produces about 11 N of thrust:
$$T = \frac{2 \eta P}{g_0 I_{sp}} \approx \frac{2 \times 0.65 \times 250,000}{9.81 \times 3000} \approx 11, \text{N}$$
The propellant mass needed for a given Δv follows the rocket equation. For a final mass of about 137.5 t (rock + sail + bus + SEP dry), the propellant is:
| Δv | Propellant fraction | Propellant mass | Total initial mass |
|---|---|---|---|
| 2 km/s | 6.6% | ~9 t | ~147 t |
| 4 km/s | 12.8% | ~18 t | ~155 t |
Those fractions are modest because high-Isp electric propulsion is efficient. The price is time: a pure 11-N burn on a ~140 t stack gives only about 0.08 mm/s² of acceleration, so a 2 km/s manoeuvre takes roughly a year of continuous thrust, and 4 km/s takes about two years. The sail, when it can be aligned usefully, roughly doubles the thrust and halves those times.
Tug dry mass
| Item | Mass |
|---|---|
| Sail membrane and booms | 15 t |
| SEP dry (250 kW) | 7.5 t |
| Bus, power, comms, attitude | 10 t |
| Capture tether / grapple | 5 t |
| Tug dry total | 37.5 t |
4. The comparison
For a 100 t rock, the tug dry mass is about 38 t. Add propellant and the tug starts at roughly 47–56 t. The ratio of tug to rock is therefore about 0.5. That is not a small adapter; it is a second spacecraft almost half the size of the payload.
For context, NASA’s Asteroid Redirect Mission alternate concept studied a 4.97 t SEP vehicle with up to 10 t of xenon to retrieve a multi-tonne boulder. That is the same family of hardware, just scaled up by an order of magnitude in rock mass and by a factor of several in sail area.
5. The Clarke echo
Clarke’s The Fountains of Paradise is about building a space elevator whose own mass eventually dwarfs the payload it was meant to lift. The hybrid minimoon tug has the same feel: the engine becomes a project comparable to the cargo. The question stops being “can we move the rock?” and becomes “can we afford to build and launch the engine?”
6. What this changes
The 100 t minimoon capture is not a CubeSat ride-along. A plausible tug masses tens of tonnes, carries a square-kilometre sail, and needs a multi-hundred-kilowatt electric propulsion plant. The technology pieces — large deployable membranes, high-power SEP, autonomous grapple — each exist at smaller scale, but integrating them for a single rock is a flagship-class mission, not a technology demo.
The economics therefore depend on launch cost and reusability. If the tug can be reused for multiple rocks, the per-rock capital cost falls. If it is single-use, the captured material has to be extraordinarily valuable.
7. Next curiosity
Could the same tug architecture be reused across several minimoons, amortising its enormous sail and SEP plant? Or does each capture consume enough of the tug’s life — sail ageing, propellant, structural load cycles — that reuse is illusory?